As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. In addition, secondary batteries have recently been used as power sources for electric vehicles (EVs), hybrid electric vehicles (HEVs), and the like. Accordingly, research into secondary batteries that can meet a variety of demands is underway and, in particular, demand for lithium secondary batteries having high energy density, high discharge voltage and high output stability is increasing.
Conventionally, a lithium cobalt composite oxide having a layered structure is generally used as a cathode active material of a lithium secondary battery. When such lithium cobalt composite oxide is used as a cathode active material, however, cobalt as a main component is very expensive, and the layered structure thereof undergoes changes in volume according to repeated intercalation and deintercalation of Li ions and collapses when more than half of the Li ions are deintercalated. Thus, lithium secondary batteries including such cathode active materials are not suitable for use in EVs or large capacity power storage devices in terms of safety.
In addition, a battery including a lithium manganese composite oxide having a spinel structure is not suitable for use as an energy source for EVs requiring relatively high energy density since a travel distance thereof is determined by battery electric energy.
Meanwhile, crystalline graphite is mainly used as an anode active material, which has a very low discharge potential of about −3 V with respect to a standard hydrogen electrode potential, and exhibits very reversible charge/discharge behavior due to uniaxial orientation of a graphene layer and thus has excellent cycle lifespan.
However, such crystalline graphite has poor output properties and thus a secondary battery including such anode active material is not suitable for use as an energy source for HEVs requiring high output.